Photodynamic therapy (PDT) is a method for treating cancers and other diseased tissues. In photodynamic therapy, a photosensitizer drug is generally administered to the patient and the disease site to be treated is then exposed to light, however, in the presence of endogenous photosensitizers, the administration of a photosensitizer drug may not be absolutely necessary for performing PDT. If the site is internal, it is necessary to expose the site using an endoscopic probe that contains a fiber optic cable. Once exposed to light, the photosensitizer drug (or endogenous photosensitizer) is excited and interacts with molecular oxygen to create toxic species that include singlet oxygen. Singlet oxygen reacts with the tissues and causes cell damage which kills or alters the cells. A significant limitation of photodynamic therapy is that healthy tissue as well as diseased or abnormal tissue is affected. Too much illumination will damage normal tissue while insufficient illumination will result in inadequate treatment.
Therefore, it is important that the various factors affecting the therapy be carefully controlled to ensure optimal treatment. Factors affecting photodynamic therapy include the photosensitizer drug dose, or the quantity of endogenous photosensitizers present, the light dose, pharmacokinetics of the drug, distribution of the drug in the tissue, distribution of the light in the tissue, and oxygen supply. All of the above factors are subject to biological variations in individual patients as well as variations in the disease and the specific disease site. The dosimetry parameters derived from in vitro measurements, animal studies, theoretical modelling, and prior experience with other patients may not be optimal for any given patient. At present, there is no effective method in clinical practice for determining when the treatment site has received its optimal light exposure.